Hi Van,
You need to goto SE18 -> give 'ME_PROCESS_PO_CUST' in BADI Name - > Click on Enhancement Implementation at the header -> Create -> Give some implementation name starts with 'Z ->provide some short description -> Here you need to click on 'Interface' tab -> and double click on the method which serves your purpose.
I think you need to do the enhancement for storage location so you need to use the method 'PROCESS_ITEM'.
Reward me if it helpful.
Thanks,
Siva